Exhaust gas purifying apparatus

ABSTRACT

An exhaust gas purification apparatus includes an oxidation catalyst provided in a passage through which exhaust gas flows, a urea decomposition accelerator, a selective catalytic reduction catalyst provided downstream of the urea decomposition accelerator and a urea water supplying device for supplying urea water to the urea decomposition accelerator. The urea decomposition accelerator is provided downstream end surface of the oxidation catalyst and has at least one of hydrophilic function and hydrolytic catalytic function.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust gas purification apparatus and, more specifically, to an exhaust gas purification apparatus having a urea selective catalytic reduction (hereinafter referred to merely as SCR) system for reducing nitrogen oxides (NOx) in exhaust gas emitted from a diesel engine.

The urea SCR system has been developed for reducing NOx in exhaust gas emitted from a diesel engine. The urea SCR system employs an SCR catalyst for converting NOx into nitrogen (N2) and water (H2O) by chemical reaction between NOx and ammonia (NH3) generated by hydrolysis of urea water.

The SCR catalyst is provided in the exhaust passage between the engine and the muffler. Furthermore, an oxidation catalyst and an injection valve for injecting urea water into the exhaust gas are provided upstream of the SCR catalyst. The oxidation catalyst oxidizes hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas into water (H2O) and carbon dioxide (CO2) and also promotes the oxidation of nitrogen oxide (NO) into nitrogen dioxide (NO2). Another oxidation catalyst is provided downstream of the SCR catalyst for promoting the oxidation of ammonia unreacted with NOx so as to prevent emission of the unreacted ammonia into the atmosphere.

A diesel particulate filter (hereinafter referred to merely as DPF) is also provided in the exhaust passage between the engine and the muffler for reducing particulate matter (PM) such as carbon in the exhaust gas. The exhaust gas purification apparatus including the urea SCR system and the DPF has many components provided between the engine and the muffler and requires a large space for mounting of such components to a vehicle. Therefore, a downsized urea SCR system has been proposed for facilitating the installation of the system in the vehicle.

Published Japanese Translation 2001-511494 of PCT International Publication discloses an exhaust gas purification apparatus that includes a mixing device functioning as a gas-guiding device, an injection device for injecting urea water as a reducing agent and a catalytic device provided downstream of the injecting device and including a hydrolytic catalytic module and an SCR catalytic module. The hydrolytic catalytic module is provided upstream of the SCR catalytic module in the catalytic device. The same Published Japanese Translation discloses another exhaust gas purification device in which a second mixing device is provided between the injecting device and the catalytic device.

The exhaust gas purification apparatus of the above Published Japanese Translation has accomplished the improvement of the efficiency of chemical reaction by the catalytic module in the catalytic device by ensuring uniform distribution of the reducing agent in the exhaust gas with the aid of the exhaust gas flow caused by the mixing device for reducing NOx in the exhaust gas effectively. In addition, the exhaust gas purification apparatus achieves reduction of the distance between the injection device and the catalytic device and of the structural space of the apparatus. However, for ensuring the time that is long enough for urea water to be hydrolyzed, the distance that the injected urea water moves before reaching catalytic device should be long so that the time for urea water or the reducing agent to stay upstream of the catalytic device is long enough for the hydrolysis of urea water. When the structural space of the exhaust gas purification apparatus is reduced, urea water is supplied to the SCR catalytic module without being hydrolyzed sufficiently into ammonia. The exhaust gas purification apparatus improves the efficiency of hydrolysis by providing the hydrolytic catalytic module in the catalytic device.

However, when the structural space of the exhaust gas purification apparatus is reduced, it is difficult for the apparatus to provide a distance between the hydrolytic catalytic module and the SCR catalytic module that is long enough to ensure the reaction time for urea water to be hydrolyzed. Therefore, if the structural space of the exhaust gas purification apparatus attempted to be reduced, the quantity of unreacted urea water supplied to the SCR catalytic module without being hydrolyzed into ammonia increases, with the result that the efficiency of the reduction of NOx in comparison to urea water usage deteriorates.

The present invention is directed to providing an exhaust gas purification apparatus for improving the efficiency of the reduction of NOx in comparison to urea water usage.

SUMMARY OF THE INVENTION

An exhaust gas purification apparatus includes an oxidation catalyst provided in a passage through which exhaust gas flows, a urea decomposition accelerator, a selective catalytic reduction catalyst provided downstream of the urea decomposition accelerator and a urea water supplying device for supplying urea water to the urea decomposition accelerator. The urea decomposition accelerator is provided downstream end surface of the oxidation catalyst and has at least one of hydrophilic function and hydrolytic catalytic function.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic view showing an exhaust gas purification apparatus according to a first embodiment of the present invention and its associated components;

FIG. 2 is a schematic cross sectional view of the exhaust gas purification apparatus of FIG. 1; and

FIG. 3 is a schematic cross sectional view of an exhaust gas purification apparatus according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following will describe the embodiments of the exhaust gas purification apparatus according to the present invention with reference to FIGS. 1 through 3. Referring to FIGS. 1 and 2 showing the first embodiment, the exhaust gas purification apparatus which is designated generally by 101 and its associated components will be described. The exhaust gas purification apparatus 101 is employed in a vehicle equipped with a diesel engine.

Referring to FIG. 1, an engine assembly including an engine 1 and the exhaust gas purification apparatus 101 is designated generally by reference numeral 10. The engine 1 has a plurality of cylinders 1A each having an intake port 1B to which an intake manifold 4 is connected for distributing intake air to the respective cylinders 1A. The intake manifold 4 has an inlet 4A to which an engine intake pipe 3 is connected and the engine intake pipe 3 is further connected to a compressor housing 8A of a turbocharger 8. The compressor housing 8A is connected to an intake pipe 2 through which outside air is introduced.

On the other hand, an exhaust manifold 5 is connected to a plurality of exhaust ports 1C of the engine 1 for collecting exhaust gas emitted from the respective exhaust ports 1C. An outlet 5A of the exhaust manifold 5 is connected to a turbine housing 8B of the turbocharger 8, to which the exhaust gas purification apparatus 101 having a substantially cylindrical shape is connected and disposed adjacent to a lateral side of the engine 1. The exhaust gas purification apparatus 101 is connected to an exhaust pipe 6, the downstream end of which is further connected to a muffler 7. The intake pipe 2, the turbocharger 8, the engine intake pipe 3 and the intake manifold 4 cooperate to form an intake system of the vehicle, while the exhaust manifold 5, the turbocharger 8, the exhaust gas purification apparatus 101, the exhaust pipe 6 and the muffler 7 cooperates to form an exhaust system of the vehicle. The engine 1, the engine intake pipe 3, the intake manifold 4, the exhaust manifold 5 and the turbocharger 8 cooperate to form the aforementioned engine assembly 10.

Referring to FIG. 2, the exhaust gas purification apparatus 101 includes a casing 11 having a substantially cylindrical shape. The casing 11 has an upstream end face 11A to which the outlet 8B2 of the turbine housing 8B of the turbocharger 8 is connected and a downstream end face 11B to which the upstream end 6A of the exhaust pipe 6 is connected. The casing 11 communicates internally with the turbine housing 8B and the exhaust pipe 6.

The cylindrical casing 11 houses therein an oxidation catalyst layer 12 supporting an oxidation catalyst and a diesel particulate filter (DPF) 14 as a particulate matter collecting device disposed downstream of the oxidation catalyst layer 12 with respect to the flow of exhaust gas in the casing 11. The oxidation catalyst layer 12 and the DPF 14 are made in the form of a layer extending perpendicular to the axis of a cylindrical portion 11C of the casing 11 over the entire radial dimension of the interior of the cylindrical portion 11C. The oxidation catalyst layer 12 and the DPF 14 are disposed spaced apart each other thereby to form therebetween a space 16.

The oxidation catalyst layer 12 supports thereon the oxidation catalyst for oxidizing hydrocarbons (HC) and carbon monoxide (CO) into water (H2O) and carbon dioxide (CO2) and also promoting the oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2). The oxidation catalyst of the oxidation catalyst layer 12 uses material such as platinum (Pt), palladium (Pd), rhodium (Rh), silver (Ag), iron (Fe), copper (Cu), nickel (Ni), gold (Au) or a mixture of two or more of these materials.

The DPF 14 is made of a porous material such as ceramic for capturing particulate matter (PM) contained in the exhaust gas. The DPF 14 has an (urea) SCR catalyst 15 as a selective catalytic reduction catalyst supported thereon, e.g., by coating. The selective catalytic reduction catalyst serves to promote the chemical reaction selectively among specific chemical substances. The SCR catalyst 15 catalyzes the reaction between nitrogen oxide (NOx) and ammonia (NH3) thereby to reduce NOx into nitrogen (N2) and water (H2O). Material of the SCR catalyst 15 includes an oxide of zirconium (Zr), titanium (Ti), silicon (Si), cerium (Ce), or tungsten (W), a complex of these oxides and a ZSM-5 type zeolite partially replaced by a metal such as iron (Fe) and copper (Cu).

The oxidation catalyst layer 12 supports on at least a part of the downstream end surface 12B thereof with regard to the flow of exhaust gas, i.e., on the surface thereof facing the DPF 14, a hydrophilic layer 13 having a hydrophilic function and forming the urea decomposition accelerator of the invention. The hydrophilic layer 13 is formed by coating the end surface 12B of the oxidation catalyst layer 12 with a catalytic material that has a hydrolytic catalytic function for accelerating the hydrolysis and a hydrophilic function. This catalytic material having the hydrolytic catalytic function and the hydrophilic function includes a metal oxide such as silica (SiO2), alumina (Al2O3), ceria (CeO2), titania (TiO2), tungsten oxide (WO3) and the like. Material forming the hydrophilic layer 13 is made of a single metal oxide or a combination of the above metal oxides. The performance for hydrolysis of the hydrophilic layer 13 can be improved by adding silver (Ag) or platinum (Pt) other than the above metal oxides to the material forming the hydrophilic layer 13.

An injection valve 18 that is an electromagnetic valve is provided in the cylindrical portion 11C of the casing 11 at a position between the oxidation catalyst layer 12 (or the hydrophilic layer 13) and the DPF 14 (or the SCR catalyst 15). Specifically, the position is closer to the oxidation catalyst layer 12 (or the hydrophilic layer 13) than the DPF 14 (or the SCR catalyst 15). The injection valve 18 forms a urea water supplying device of the invention. The injection valve 18 is connected to a urea water tank 19 provided in a vehicle (not shown) and operable to inject urea water into the space 16 of the casing 11. The injection valve 18 is provided at a position that is adjacent to and immediately downstream of the hydrophilic layer 13 so that urea water is injected by the injection valve 18 toward the downstream end surface 12B of the oxidation catalyst layer 12, i.e., the downstream surface 13B of the hydrophilic layer 13. The injection valve 18 is electrically connected to a dosing control unit (DCU) 30 that controls the opening and closing operation of the injection valve 18. The urea water tank 19 has an electric pump for supplying urea water to the injection valve 18. The electric pump is electrically connected to the DCU 30 and the pump operation is controlled by the DCU 30.

A cylindrically-shaped mixer 17 is provided on the upstream end surface 14A of the DPF 14 for distributing substances in the exhaust gas uniformly over the end surface 14A. The mixer 17 has a structure that is similar to that disclosed in Published Japanese Translation H06-509020 of PCT international publication or Japanese Patent Application Publication 2006-9608. The mixer disclosed in Published Japanese Translation H06-509020 is made in the form of a lattice that divides the gas passage into plural cells so as to cause the gas flowing through each cell to flow spirally and also to flow toward the adjacent cell. This helps the substances in the exhaust gas to spread uniformly in the whole passage. On the other hand, the mixer disclosed in Japanese Patent Application Publication 2006-9608 has plural plates each extending perpendicularly to the direction of gas flow, which provides serpentine gas passage serving to distribute the substances in the gas uniformly.

Another oxidation catalyst layer 20 that supports oxidation catalyst for oxidizing ammonia is provided in the exhaust pipe 6 downstream of the exhaust gas purification apparatus 101. Platinum (Pt), palladium (Pd), silver (Ag), iron (Fe), copper (Cu), nickel (Ni), gold (Au) or the like may be employed as the material of the oxidation catalyst of the oxidation catalyst layer 20.

An exhaust gas temperature sensor 52 is provided upstream of the oxidation catalyst layer 12 and also downstream of the upstream end face 11A of the casing 11 for detecting the temperature of exhaust gas. The exhaust gas temperature sensor 52 is electrically connected to the DCU 30 and sends detected temperature information to the DCU 30. A first NOx sensor 51 is provided in the casing 11 at a position upstream of the exhaust gas temperature sensor 52 for detecting the NOx concentration and a second NOx sensor 53 is provided downstream of the downstream end face 11B of the casing 11, more specifically, at a position downstream of the oxidation catalyst layer 20 in the exhaust pipe 6, for detecting the NOx concentration. The first and the second NOx sensors 51, 53 are electrically connected to the DCU 30 and send information about the NOx concentration to the DCU 30. As described above, the exhaust gas purification apparatus 101 having the SCR catalyst 15 and the DPF 14 integrated together is mounted to the engine assembly 10 at a position adjacent to the engine 1 (refer to FIG. 1).

The following will describe the operation of the exhaust gas purification apparatus 101 according to the first embodiment and its associated components with reference to FIGS. 1 and 2. Referring to FIG. 1, while the engine 1 is running, outside air is flowed into the compressor housing 8A of the turbocharger 8 through the intake pipe 2. The air is pumped by a compressor wheel (not shown) in the compressor housing 8A and flowed to the engine intake pipe 3 under an increased pressure. The air is flowed into a cylinder 1A in the engine 1 through the engine intake pipe 3 and the intake manifold 4. Then, the air in the cylinder 1A is mixed with fuel (light oil) supplied into the cylinder 1A and the fuel is ignited spontaneously for combustion.

Exhaust gas produced by the combustion is discharged into the exhaust manifold 5 through a plurality of exhaust ports 1C to be collected by the exhaust manifold 5 and then flows into the turbine housing 8B of the turbocharger 8. The exhaust gas flowing through the turbine housing 8B increases rotation speed of the turbine wheel (not shown) in the turbine housing 8B and the compressor wheel connected to the turbine wheel and then is discharged into the exhaust gas purification apparatus 101. After flowing through the exhaust gas purification apparatus 101, the exhaust gas flows through the oxidation catalyst layer 20, the exhaust pipe 6 and the muffler 7 and then is discharged outside the vehicle (not shown).

Referring to FIG. 2, all the exhaust gas flowed into the exhaust gas purification apparatus 101 flows firstly through the oxidation catalyst layer 12. While the exhaust gas flows through the oxidation catalyst layer 12, hydrocarbons and carbon monoxide in the exhaust gas are oxidized into carbon dioxide and water, and part of NO is oxidized into NO2 that can be reduced easily. After flowing through the oxidation catalyst layer 12, the exhaust gas flows through the hydrophilic layer 13 and the mixer 17 and then into the DPF 14 supporting the SCR catalyst 15. PM in the exhaust gas is captured by the DPF 14.

Meanwhile, the DCU 30 activates the electric pump in the urea water tank 19 and also opens the injection valve 18 for injection of urea water from the injection valve 18 toward the hydrophilic layer 13 located upstream of the space 16.

The injected urea water is adsorbed on the surface 13B of the hydrophilic layer 13. Specifically, the urea water injected onto the surface 13B of the hydrophilic layer 13 is dispersed in radial directions of the cylindrical portion 11C of the casing 11 due to the hydrophilic property of the hydrophilic layer 13 and is adsorbed uniformly on the surface 13B.

The oxidation catalyst layer 12 has therein the heat due to the exhaust gas flowing therethrough and also the reaction heat due to the oxidation of NO and the like in the exhaust gas. The urea water adsorbed on the surface 13B of the hydrophilic layer 13 is hydrolyzed into ammonia and carbon dioxide (CO2) by the heat that the oxidation catalyst layer 12 has, the heat of the exhaust gas flowing through the hydrophilic layer 13 and also the hydrolytic catalytic function of the hydrophilic layer 13. The urea water is then adsorbed uniformly on the surface 13B of the hydrophilic layer 13 and, therefore, the reaction time required for the hydrolysis can be ensured, with the result that the urea water is hydrolyzed into ammonia effectively. Furthermore, since the urea water is dispersed and adsorbed uniformly on the surface 13B of the hydrophilic layer 13, ammonia is generated uniformly on the surface 13B of the hydrophilic layer 13.

Since the urea water dispersed and adsorbed on the hydrolytic catalyst as described above is hydrolyzed, the hydrolysis takes place with a high efficiency. Furthermore, the urea water can make use of the heat of hot exhaust gas immediately after being emitted from the turbocharger 8 of the engine 1. Therefore, the urea water can easily ensure the heat and the temperature required for the hydrolysis. Moreover, urea water is injected and hydrolyzed into ammonia in the region that is downstream of the oxidation catalyst layer 12 and, therefore, no ammonia flows into the oxidation catalyst layer 12 and is oxidized by the oxidation catalyst of the oxidation catalyst layer 12.

Ammonia generated on the hydrolysis is dispersed uniformly in radial directions of the cylindrical portion 11C of the casing 11 and flows to the mixer 17 together with the exhaust gas. Ammonia is further dispersed while flowing through the mixer 17 and then flows into the DPF 14. Ammonia that is flowed into the DPF 14 together with the exhaust gas reduces NOx contained in exhaust gas including NO and NO2 into N2 by the catalytic reaction of the SCR catalyst 15. After being dispersed uniformly on the hydrophilic layer 13, ammonia is dispersed again at the mixer 17 and then supplied uniformly to the entire DPF 14 and the SCR catalyst 15, thereby reducing NOx effectively at the SCR catalyst 15.

Unreacted ammonia which has not been used in the reduction of NOx is discharged outside the exhaust gas purification apparatus 101 together with exhaust gas.

Therefore, the exhaust gas containing residual unreacted ammonia and N2 after flowing through the DPF 14 where PM is removed is discharged from the exhaust gas purification apparatus 101 into the exhaust pipe 6. The exhaust gas thus discharged into the exhaust pipe 6 flows through the oxidation catalyst layer 20 provided in the exhaust pipe 6 and then is discharged through the muffler 7 outside the vehicle (not shown). Since the residual ammonia in the exhaust gas is oxidized and decomposed while flowing through the oxidation catalyst layer 20, no harmful ammonia is discharged outside.

The catalyst has a characteristic that it activates the catalytic action at a temperature more than a predetermined temperature. The DCU 30 is operated to open the injection valve 18 when the temperature detected by the exhaust gas temperature sensor 52 is the predetermined temperature at which the SCR catalyst 15 is activated or higher, and to close the injection valve 18 when the detected temperature is under the predetermined temperature. Thus, the DCU 30 determines whether or not NOx reduction should be performed depending on the temperature detected by the exhaust gas temperature sensor 52.

Furthermore, the DCU 30 controls the injection quantity of urea water by adjusting the opening of the injection valve 18 based on the NOx concentration detected by the first NOx sensor 51. Similarly, the DCU 30 controls the injection quantity of urea water by adjusting the opening of the injection valve 18 based on the NOx concentration detected by the second NOx sensor 53, that is the NOx concentration of exhaust gas after flowing through the SCR catalyst 15 and the oxidation catalyst layer 20. For example, when the NOx concentration detected by the second NOx sensor 53 exceeds a predetermined level, the DCU 30 increases the injection quantity of urea water by opening the injection valve 18 further. Thus, the DCU 30 adjusts the supply quantity of urea water to the SCR catalyst 15, i.e., the supply quantity of ammonia, thereby controlling the NOx reduction performance of the exhaust gas purification apparatus 101.

Referring to FIG. 1, the exhaust gas purification apparatus 101 is disposed adjacent to the engine 1 and, therefore, hot exhaust gas immediately after being emitted from the engine 1 flows into the exhaust gas purification apparatus 101 through the turbocharger 8. Furthermore, the heat generated by the engine 1 is imparted to the exhaust gas purification apparatus 101 located adjacent to the engine 1 and transmitted inward through outer wall of the casing 11.

Referring to FIG. 2, the oxidative catalyst layer 12, the hydrophilic layer 13 and the DPF 14 supporting the SCR catalyst 15 all disposed inside the casing 11 are subject to the heat of the hot exhaust gas and the heat imparted from the engine 1 and, therefore, the temperature of the respective components tends to increase. The temperature increasing rate of the respective components, i.e., the oxidation catalyst of the oxidation catalyst layer 12, the hydrophilic catalyst of the hydrophilic layer 13 and the SCR catalyst 15 in the exhaust gas purification apparatus 101 during a cold start of the engine 1 is improved and the time required for activating each catalyst is shortened. Eventually, the performance of NOx reduction is improved.

Thus, the exhaust gas purification apparatus 101 of the present invention includes the oxidation catalyst layer 12 provided in exhaust gas passage, the hydrophilic layer 13 that is provided at least on the downstream end surface 12B of the oxidation catalyst layer 12 and having at least one of the hydrophilic function and the hydrolytic catalytic function, the SCR catalyst 15 provided downstream of the hydrophilic layer 13 and the injection valve 18 for supplying urea water to the hydrophilic layer 13.

The urea water supplied to the hydrophilic layer 13 is dispersed and adsorbed on the hydrophilic layer 13 by the hydrophilic function and can make use of the heat generated by the oxidation of NO to NO2 and the heat of the exhaust gas flowing through the oxidation catalyst layer 12. Therefore, urea water is hydrolyzed very efficiently and ammonia resulting from the hydrolysis of the urea water is dispersed uniformly over the hydrophilic layer 13. Thus, the hydrolysis of urea water into ammonia is accomplished with high efficiency, which helps to improve the reaction of the ammonia in the SCR catalyst 15. The hydrolysis reaction of urea water supplied to the hydrophilic layer 13 is accelerated by the heat of the oxidation catalyst layer 12 and the heat due to the hydrolytic catalytic function of the hydrophilic layer 13, thereby improving the efficiency of the hydrolysis. Thus, the reduction of NOx in the exhaust gas purification apparatus 101 using the urea water can be improved by the hydrophilic function and the hydrolytic catalytic function of the hydrophilic layer 13. Since the efficiency of hydrolysis of urea water is improved by providing the hydrophilic layer 13, the distance between the hydrophilic layer 13 and the SCR catalyst 15 can be shortened, thereby making it possible for the exhaust gas purification apparatus 101 to be made small.

Since the injection valve 18 supplies urea water at a position downstream of the hydrophilic layer 13, neither urea water is supplied to the oxidation catalyst layer 12 nor ammonia produced by the hydrolysis of urea water flows through the oxidation catalyst layer 12. Therefore, the oxidation of ammonia into NOx by the oxidation catalyst of the oxidation catalyst layer 12 can be prevented. Due to the structure where the DPF 14 supports the SCR catalyst 15, the SCR catalyst 15 and the DPF 14 are formed integrally, thereby making it possible for the entire apparatus to be formed small. Furthermore, since the oxidation catalyst layer 12, the hydrophilic layer 13, the SCR catalyst 15 formed integrally with the DPF 14 and the injection valve 18 are all housed in the single casing 11, the entire apparatus can be made still smaller.

The exhaust gas purification apparatus 101 is mounted to the engine assembly 10 and the hot exhaust gas emitted from the engine assembly 10 is introduced into the exhaust gas purification apparatus 101. The heat that the engine assembly 10 generates in operation is transmitted inside the casing 11 of the exhaust gas purification apparatus 101. Therefore, the time for the temperature of the hydrophilic layer 13 to be increased to the level required for the hydrolysis of urea water and also for the temperature of the SCR catalyst 15 to the level required for activating the SCR catalyst 15 during a cold start of the engine can be shortened, with the result that the NOx reduction performance can be improved.

The exhaust gas purification apparatus 102 according to a second embodiment of FIG. 3 is made by modifying the DPF 14 supporting the SCR catalyst 15 of the exhaust gas purification apparatus 101 according to the first embodiment. The following description will use the same reference numerals for the common elements or components in the first and the second embodiments, and the description of such elements or components will be omitted.

Referring to FIG. 3, the oxidation catalyst layer 12 having on the downstream side thereof the hydrophilic layer 13, an SCR catalyst layer 25 supporting the SCR catalyst and the DPF 24 are provided in this order in the downstream direction in the casing 11 of the exhaust gas purification apparatus 102. The oxidation catalyst layer 12 and the SCR catalyst layer 25 are disposed across the space 16 and the SCR catalyst layer 25 and the DPF 24 are disposed adjacent to each other. The mixer 17 is provided on the upstream end surface 25A of the SCR catalyst layer 25.

The exhaust gas introduced into the casing 11 flows through the mixer 17 after flowing through the oxidation catalyst layer 12 and the hydrophilic layer 13. NOx contained exhaust gas is reduced into N2 in the SCR catalyst layer 25, PM contained in exhaust gas is captured in the DPF 24 and the resulting exhaust gas is discharged outside the exhaust gas purification apparatus 102.

The rest of the structure and the operation of the exhaust gas purification apparatus 102 according to the second embodiment is the same as those of the exhaust gas purification apparatus 101 according to the first embodiment. The description of such structure or operation will be omitted.

The exhaust gas purification apparatus 102 according to the second embodiment offers the same advantageous effects as the exhaust gas purification apparatus 101 according to the first embodiment.

When PM is burned in the DPF 24 in the exhaust gas purification apparatus 102 for preventing the accumulation of PM, the influence of the combustion heat on the SCR catalyst of the SCR catalyst layer 25 is reduced as compared with the exhaust gas purification apparatus 101 according to the first embodiment. The exhaust gas purification apparatus 102 reduces the deterioration of the catalytic function of the SCR catalyst layer 25 due to the heat caused by burning PM and improves the durability of the SCR catalyst layer 25.

The exhaust gas purification apparatuses 101 and 102 according to the first and the second embodiments are provided in the engine assembly 10 having the turbocharger 8, respectively, but the present invention is not limited to this structure. When the engine assembly 10 dispenses with the turbocharger 8, the exhaust gas purification apparatuses 101 and 102 may be directly connected to the outlet 5A of the exhaust manifold 5, respectively. The exhaust gas purification apparatuses 101 and 102 may be provided spaced apart from the engine assembly 10, respectively.

In the second embodiment, the oxidation catalyst layer 12, the SCR catalyst layer 25, the DPF 24 and the injection valve 18 are all provided in the casing 11 of the exhaust gas purification apparatus 102, but the present invention is not limited to this structure. For example, only the DPF 24 may be provided outside the casing 11 separately from the other components.

In the first and second embodiments, the oxidation catalyst layer 20 is provided separately from the exhaust gas purification apparatuses 101, 102, but the present invention is not limited to this structure. The oxidation catalyst layer 20 may be provided inside the casing 11 downstream of the DPF 14, 24 of the exhaust gas purification apparatuses 101, 102, respectively.

The injection valve 18 is provided downstream of the oxidation catalyst layer 12 so as to supply urea water to the hydrophilic layer 13 in the exhaust gas purification apparatuses 101, 102 according to the first and the second embodiments, respectively, but the present invention is not limited to this structure. The injection valve may be so arranged that urea water is supplied toward the upstream side of the oxidation catalyst layer 12. The supplied urea water can be hydrolyzed while flowing through the oxidation catalyst layer 12 and, therefore, the efficiency of the hydrolysis of urea water can be improved. Since urea water is dispersed while flowing through the oxidation catalyst layer 12, urea water can be dispersed and adsorbed more uniformly on the hydrophilic layer 13. Accordingly, ammonia produced by the hydrolysis of urea water can be dispersed and be supplied to the SCR catalyst more uniformly. Thus, the efficiency of the reduction of NOx by ammonia in the SCR catalyst is improved.

Although the hydrophilic layer 13 is supported on part of the downstream end surface 12B of the oxidation catalyst layer 12 in the first and the second embodiments, the hydrophilic layer 13 may be supported on the entire end surface 12B of the oxidation catalyst layer 12.

The single hydrophilic layer 13 having the hydrolytic catalytic function and the hydrophilic function is used as the urea decomposition accelerator in the first and the second embodiments, but the present invention is not limited to this structure. The single hydrophilic layer serving as the urea decomposition accelerator may be formed of two different layers, one layer of which is a hydrophilic layer made of a material having only the hydrophilic function and the other layer of which is a hydrolytic catalytic layer made of a material having only the hydrolytic catalytic function for accelerating the hydrolysis of urea water. In this case, the hydrophilic layer should preferably be provided downstream of the hydrolytic catalytic layer, that is on the side facing the DPF 14 and 24 of the first and the second embodiments, respectively.

The urea decomposition accelerator may be formed of only the hydrophilic layer made of the material having the hydrophilic function. Since urea water dispersed and adsorbed on the hydrophilic layer can make use of the heat of the oxidation catalyst layer 12, urea water is hydrolyzed efficiently and the resulting ammonia is dispersed uniformly over the entire hydrophilic layer. On the other hand, the urea decomposition accelerator may be formed of only the hydrolytic catalytic layer made of the material having the hydrolytic catalytic function. In this case, urea water making use of the heat of the oxidation catalyst layer 12 is subject to the hydrolytic catalytic function by the hydrolytic catalytic layer and, therefore, the efficiency of the hydrolysis can be improved.

The casing 11 of the exhaust gas purification apparatuses 101, 102 according to the first and the second embodiments, respectively is cylindrically-shaped, but the casing 11 according to the present invention is not limited to this shape. The casing 11 may be formed with a cross-section including a prism such as quadratic prism, a sphere or an ellipsoid.

Furthermore, the mixer 17 may be dispensed with in the first and the second embodiments. 

1. An exhaust gas purification apparatus comprising: an oxidation catalyst provided in a passage through which exhaust gas flows; a urea decomposition accelerator, wherein the urea decomposition accelerator is provided downstream end surface of the oxidation catalyst and has at least one of a hydrophilic function and a hydrolytic catalytic function; a selective catalytic reduction catalyst provided downstream of the urea decomposition accelerator; and a urea water supplying device for supplying urea water to the urea decomposition accelerator.
 2. The exhaust gas purification apparatus according to claim 1, wherein the urea water supplying device supplies urea water toward downstream surface of the urea decomposition accelerator.
 3. The exhaust gas purification apparatus according to claim 1, wherein the urea water supplying device is an injection valve provided at a position that is between the urea decomposition accelerator and the selective catalytic reduction catalyst and closer to the urea decomposition accelerator than the selective catalytic reduction catalyst.
 4. The exhaust gas purification apparatus according to claim 1, further comprising: a particulate matter collecting device for capturing particulate matter contained in the exhaust gas, wherein the particulate matter collecting device is formed integrally with the selective catalytic reduction catalyst.
 5. The exhaust gas purification apparatus according to claim 4, wherein the particulate matter collecting device is provided downstream of the selective catalytic reduction catalyst.
 6. The exhaust gas purification apparatus according to claim 4, further comprising: a mixer provided on upstream end surface of the particulate matter collecting device or the selective catalytic reduction catalyst for distributing substances in the exhaust gas over the end surface of the particulate matter collecting device or the selective catalytic reduction catalyst.
 7. The exhaust gas purification apparatus according to claim 1, further comprising: a casing housing the oxidation catalyst, the urea decomposition accelerator, the selective catalytic reduction catalyst and the urea water supplying device.
 8. The exhaust emission purification apparatus according to claim 1, further comprising: an exhaust gas temperature sensor provided upstream of the oxidation catalyst for detecting a temperature of the exhaust gas; a first NOx sensor provided upstream of the oxidation catalyst for detecting NOx concentration; a second NOx sensor provided downstream of the selective catalytic reduction catalyst for detecting NOx concentration; and a dosing control unit electrically connected to the first and the second NOx sensors, the exhaust gas temperature sensor and the urea water supplying device, wherein, when the temperature detected by the exhaust gas temperature sensor is as high as a temperature at which the selective catalytic reduction catalyst is activated, the dosing control unit activates the urea water supplying device to supply urea water, and when the temperature detected by the exhaust gas temperature sensor is under the temperature at which the selective catalytic reduction catalyst is activated, the dosing control unit activates the urea water supplying device to stop supplying urea water, wherein the dosing control unit controls supply quantity of urea water based on NOx concentrations detected by the first and the second NOx sensors.
 9. The exhaust gas purification apparatus according to claim 1, wherein the exhaust gas purification apparatus is fixed to an engine assembly.
 10. The exhaust gas purification apparatus according to claim 1, wherein the urea decomposition accelerator is formed by coating the downstream end surface of the oxidation catalyst with a material that has a hydrolytic catalytic function and a hydrophilic function.
 11. The exhaust gas purification apparatus according to claim 10, wherein the material includes at least one of silica (SiO2), alumina (Al2O3), ceria (CeO2), titania (TiO2) and tungsten oxide (WO3). 